I. Field of the Invention
This invention relates to a method and apparatus for controlling the temperature or degree of cooling of an ingot during casting. More particularly, the invention relates to a method and apparatus for controlling the skin temperature or degree of cooling of an ingot during start-up of casting.
II. Background Art
During direct chill (DC) casting of aluminum and aluminum alloys in an open-ended mould, the cast ingot is subjected to three kinds of cooling, i.e. primary cooling (heat is extracted through the mould face), secondary cooling (the temperature of the ingot surface is controlled by spraying or injecting using a liquid jet coolant on the solidified surface of the ingot as it exits from the casting mould) and bottom block or stool cap cooling (heat is extracted through a solid starting block used to initiate the casting process). As primary cooling and bottom block cooling are not easily modified or adjustable in time, the secondary cooling is the main means of affecting the skin temperature profile of the cast ingot or the degree of cooling as it is being formed. Proper control of secondary cooling is required to prevent a variety of casting problems and ingot defects. Excessive or insufficient cooling may be responsible for ingot defect formation such as excessive butt curl, resulting in severe bleeding on short sides of rectangular cross-section ingots, cold folding requiring extra scalping before hot rolling, and cracking caused by excessive thermo-mechanical stresses.
This problem is particularly acute during the start-up phase of such a DC casting process, as the thermal and mechanical behaviour of the ingot butt is in a transient phase which can cause high thermal and mechanical stresses. Using the combined effect of the three types of cooling, but particularly through modification of the cooling effect of secondary cooling, the aim has been to progressively increase the ingot cooling until a steady state is reached without causing product defect formation. It has not been possible to determine quantitatively the combined cooling effect of all individual parameters influencing ingot cooling. Consequently, corrective actions have often been based on the personal experience of the operator, or by trial and error. This often renders process control very difficult, particularly when there is a lack of experience with new alloys, products or new casting technologies.
A variety of techniques are known for the modification of the cooling effects of the secondary cooling, such as "pulsed water" as disclosed in U.S. Pat. No. 3,441,079, or gas injection as disclosed in U.S Pat. No. 4,693,298. However, these techniques have generally been pre-programmed and do not use any feedback of the ingot condition to modify the application of the control. Therefore, they are unable to respond to variations in the coolant properties, mould cooling variations, metal temperature, casting speed, etc.
German patent DE 1,941,816 (assigned to VAW) proposes the use of a temperature measurement probe which contacts the ingot surface as a means to provide direct temperature feedback to control the coolant application. The probes are attached to a wheel device which allows contacts to be made as the ingot descends. It is disclosed that the device is used to control coolant flow to compensate for heat flux changes and changes in coolant properties. Such a temperature measurement probe consists of a pair of contacts of differing metals which, on making contact with the ingot surface, generates a thermal electromotive force (emf) which is interpreted as the surface temperature at that location. However, this means that the measurement technique relies on uniform surface contacts since there is no means of verifying measurement validity, particularly when the temperature varies along the cast ingot, for example at the start of casting. Because of the size of the wheel, the device has limitations as to where it can be located and the necessarily large spacing between contact points.
There is therefore a need to be able to reliably measure and control the ingot surface temperature as the ingot emerges from the mould even in the early stages of casting when the temperature varies substantially and, furthermore, there is a need for a method of controlling coolant delivery or properties so that it will permit reliable early-stage casting of ingots that are prone to crack during start-up.